1. Field of the Invention
This invention relates to a ground isolation circuit for electronic circuitry and, more particularly, to a ground isolation circuit for isolating a plurality of signal channels such as in motor vehicle-mounted stereophonic sound equipment.
2. Description of the Prior Art
In motor vehicle-mounted stereophonic sound equipment to be encountered with enormous noise radiations, it is common to employ shielded cables for wiring discrete electronic circuits, such as a compact disc (CD) player and a main amplifier, and shielding braids of the shielded cables are grounded to the ground of each electronic circuit at both ends in order to protect the electronic circuits from external noise.
The external noise may be eliminated by connecting the electronic circuits with shielded cables, as mentioned above, however, this may cause another problem, a so-called "ground loop noise", in such shielding system, since a ground loop is formed by the grounded shielding braids between the connected electronic circuits and, if there is a difference in ground potential, a loop current may flow through the formed ground loop.
Accordingly, to eliminate the ground loop noise caused by the ground potential difference between the connected electronic circuits, in case the connection of which is done by employing the shielded cables, a differential amplifier is normally interposed at the input side of the electronic circuit at the post-stage, as a component of the ground isolation circuit, for isolating the grounds of the connected electronic circuits from each other.
Referring to FIG. 1, there is shown, for example, a prior-art-type ground isolation circuit for use in connecting CD player unit with an amplifier in a motor vehicle-mounted stereophonic sound equipment. As known, there are a number of noise radiating sources in the motor vehicle, such as an engine, an ignition system, a microcomputer for controlling the vehicle and the like, and radiated external noises are easily picked up by a wire harness installed within a body of the motor vehicle for connecting discrete electronic units.
Consequently, a RCA cord 5, comprising elements of which are shielded independently by shielding braids 4L and 4R, has been employed in such motor vehicle wiring for transmitting left (L) and right (R) channel signals of the stereophonic sound through shielded signal lines of 3L and 3R.
In FIG. 1, the signal line 3L of the RCA cord 5 for the L channel signal is wired between an output terminal L1 of the L channel signal of the CD player unit 1 and an input terminal L2 of the L channel signal of the amplifier unit 2. In like manner, the signal line 3R of the R channel is wired between an output terminal R1 of the R channel signal of the CD player unit 1 and an input terminal R2 of the R channel signal of the amplifier unit 2. Further, the shielding braids 4L and 4R of the RCA cord 5 are combined, respectively, into one braid at the both sides of the CD player unit 1 and amplifier unit 2 and are grounded respectively to ground terminals G1 and G2 of the units 1 and 2.
On the other hand, differential amplifiers 6L and 6R, such as operational amplifiers, are interposed between the L channel signal input terminal L2 and the ground terminal G2 of the amplifier unit 2 and between the R channel signal input terminal R2 and the ground terminal G2, respectively, constituting a so-called ground isolation circuit. In FIG. 1, resistors 7 through 14 are biasing resistors for each differential amplifier.
In the example shown in FIG. 1, for the ease of understanding, the CD player unit 1 is shown symbolically by simply designating signal sources 15L and 15R and inner resistors 16L and 16R for the L and R channels. However, it is apparent for those skilled in the art that a photo pick-up unit, head amplifiers and the like are included in the CD player unit 1.
Stated hereinafter is a summary of the reason why the ground loop noise is generated in the circuit of FIG. 1.
If a potential difference of Vn is generated between the grounds of the CD player unit 1 and the amplifier unit 2 for any reason, loop currents may flow through the signal lines 3L and 3R, and the shielding braids 4L and 4R based on the generated potential difference Vn. The loop current that flows through each loop circuit has the same phase and is called a common mode current. In general, the cause of the noise is not the common mode current, but a normal mode current of opposite phase. Since the current phase of the common mode current is in phase, cancellation takes place within the current loop, thus resulting in no ground-loop noise.
In an actual circuit arrangement, however, it is common that impedances of the signal lines 3L and 3R and the shielding braids 4L and 4R differ from each other. Because of this fact, if electronic circuits at the pre-stage and the post-stage are connected without taking any measures against the noise problem, a difference in the common mode current is transferred into a normal mode, and which noise in turn enters into the signal lines as the ground loop noise.
To eliminate the problem encountered by the ground loop noise, it has been proposed, as shown in FIG. 1, that differential amplifiers 6L and 6R are interposed between the signal terminals and ground terminals of the respective channels in the electronic circuit at the post-stage, and each channel signal is applied between a non-inverted input terminal (+) and an inverted input terminal (-) of each differential amplifier. In this circuit arrangement, an amount of the common mode current that flows through the signal line 3L will be identical to that of the shielding braid 4L and that an amount of the common mode current that flows through the signal line 3R will be identical to that of the shielding braid 4R. Therefore, by subtracting the common mode current from the other having the same value at respective differential amplifiers 6L and 6R, it is possible to cancel out the amount of difference between the common mode currents that flow through the signal lines and the shielding braids. In this way, there will be no transfer of the common mode current to the normal mode current, this in turn prevents the ground loop noise from occurring.
As seen in FIG. 1, however, if the electronic circuits at the pre-stage and the post-stage are connected by utilizing shielded connecting cables, such as the RCA cord 5 wherein every cable is shielded independently for each channel, there will be formed a DC loop by the shielding braid 4L for the L channel and the shielding braid 4R for the R channel. Therefore, if an external radiation noise interlinks with this DC loop, there will be induced a noise current that circulates the DC loop as shown by the arrow in FIG. 1.
A voltage caused by the induced noise current is then added in series with a voltage across the input terminals (+) and (-) of each of the differential amplifiers 6L and 6R, and acts upon the differential amplifiers to cause them to circulate the normal mode currents. Consequently, there has been a problem such that the induced noise by the normal mode current enters into the signal line for each channel as an interfering noise. This phenomenon causes a serious problem for motor vehicle-mounted audio equipment.
It is therefore an object of this invention to eliminate the problems encountered by the prior art ground isolation circuits and to provide a ground isolation circuit capable of preventing an induced noise by external radiation noises even when inter-connections are made between two electronic circuits at the pre-stage and the post-stage by employing shielded connecting cables for respective channels such as a RCA cord.